Accurate evapotranspiration (ET) data is essential for determining the depth of water to be applied through irrigation systems. At present, data from weather stations are used in empirical equations to predict evapotranspiration which is not site/crop specific and is prone to large errors. Alternatively, pan evaporation measurements and evaporation from porous plates (atmometers) have been used to measure the depth of evaporation and relate this measurement to ET using crop coefficients. However, plant evapotranspiration is controlled not only by weather conditions but also by soil water content. The atmometer, pan evaporation, and the weather-data based methods of predicting ET are not affected by soil water content and therefore do not take into account the plant response to soil moisture deficit as shown in the constant rate in FIG. 1. This error might lead to an over application of irrigation water.
Atmometers and pan evaporation meters have been used for over 50 years as a method to measure evaporation rate as affected by weather conditions. As noted above, this evaporation rate is multiplied by a crop coefficient to estimate evapotranspiration from a crop. When the soil is wet, the crops evapotranspire at the maximum rate just like the evaporation from an atmometer or a pan evaporation meter characterized by the weather limiting phase as shown in FIG. 1. However, when the soil becomes dry, the plants find it difficult to draw water from the soil leading to a decrease in evapotranspiration due to increased stomatal resistance.
Consequently, the plants tend to lower their ET rate by stomatal closure (wilting) to a rate to match decreased flow of water towards the roots through the dry soil within the root zone. This phenomenon is shown in FIG. 1 as a declining part of the graph located within the soil-limiting phase. However, the traditional atmometer or the pan evaporation meter is unresponsive to soil water content. Therefore, any estimation of ET based on evaporation from conventional atmometers will lead to an over-estimation of the ET. This will lead to over-application of irrigation water for crops, gardens, or golf courses.